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Written By: Jeff 


Project video at 9041 1(5>N07... 

For my son's fourth birthday, we decided on a fire truck theme. We started the party with a 
tour of a fire station and then went back to the house for a firefighters' obstacle course and 
the creation of several Duplo fire trucks (and of course, cake, ice cream, and fellowship). In 
keeping with the theme, my wife made several firefighter-themed treats and props. 

Being largely unable to bake, my most significant party theme contribution was repurposing 
my old car launcher invention ( 1(5)N07... ) to be used as 
a MatchBox-scale fire station. 

I originally made the car launcher to live at the bottom of our coat closet and accept car- 
dispensing commands from distant relatives via our family blog. For our fire-themed party, I 
mounted it in the bottom of our living room entertainment center and built a cardboard fire 
station facade. I swapped out the old PHP/Processing/AppleScript/BS2 control scheme for a 
an Arduino-based radio-monitoring scheme. I mounted a trunk-tracking radio scanner (from 
one of my other hobbies, amateur radio) on the underside of the car launcher. I programmed 
the scanner to scan the Alert talk groups for our county fire department, our city fire 
department, and our county emergency medical service. The Alert talk groups are just for 
relaying dispatch orders with a computer-synthesized voice. No human voices are 
transmitted there, just a brief alert warble followed by the orders to get moving. Sticking to 
merely the fire department talk groups would have been ideal, but there might not have been 
enough fire calls during the party to dispatch all of our toy trucks. 

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I programmed the Arduino to monitor the voltage of the scanner's audio and operate the fire 
truck launcher when a call comes through. I have the scanner's internal speaker muted and 
the Arduino controls a relay that feeds an external speaker only when a call comes through 
for which it is dispatching a truck. I have a minimum-time-between-launches set in the code 
so the launcher doesn't empty itself too fast. The launcher uses a single continuous-rotation 
servo to open a door that is mounted on a drawer slide. An IR emitter and detector are used 
to sense each stopping point, and small switches are used to detect full open and full close. 

I mounted a couple of red LEDs on the cardboard fire station facade to add to the effect and 
to warn nearby partygoers that a fire truck is forthcoming. 

The net effect of all this is that when a real fire truck or ambulance is dispatched in Wake 
County, our tiny fire station plays the same radio call the firefighters hear, at the same time 
they hear it, and shoots out a toy fire truck into our living room for the entertainment of our 
party guests. 

Step 1 — Dispatchatron 


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• Gravity gets the cars moving. The 
launching mechanism just moves 
the door out of the way. 

• I used a fancy trunk-tracking 
scanner to monitor several talk 
groups. In my area, the emergency 
services use atrunking radio 
system. They also simulcast the 
alert talk groups on a single VHF 
frequency, though, so I could have 
used a simpler radio if I had one. 

• A fixed continuous-rotation servo 
controls the sliding drawer by 
moving the VEX chain through the 
gears, stopping when the IR pair 
detects a stop or when the limit 
switches detect full open or close. 

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Step 2 

• I whipped up the facade out of 
cardboard and construction paper. I 
mounted LEDs on either side of the 
door for a nice flashing effect. The 
sign you see to the left is one of 
many we had to give various food- 
and entertainment-related 
information to partygoers. 

Step 3 

• Here is my Dispatchatron mounted 
to the back side of the 
entertainment center. I'm a tad 
embarrassed by my sloppy wiring, 
but I had to make sure it was ready 
in time for the birthday party. I got 
a majorly sore back reaching back 
in here for troubleshooting and the 
addition of the LEDs. 

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Step 4 

• After installing the Dispatchatron 
behind the entertainment center 
and getting all the various 
alignments and clearances perfect, 
I realized that the Arduino was not 
driving my cardboard facade's 
LEDs brightly enough to be seen 
during the day. I threw together a 
couple PNP transistor boosters to 
reach back and plug in. I ran the 
red wire off the emitter to plug into 
my +5v rail, a 10k resistor off the 
base to plug into the Arduino 
output, and a snipped-off piece of 
female header into which I plugged 
the wire going to the LED. 

This document was last generated on 2012-11-01 06:02:26 PM. 

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